The present invention relates to tubular electric heating elements.
Tubular electric heating elements are commonly used in domestic appliances such as ovens, ranges, toasters and broilers but also have a wide variety of industrial applications.
The tubular heating element is formed of a generally tubular metal sheath serving as the casing. The generally tubular sheath can have any one of a variety of cross-sectional shapes, including circular, oval, rectangular, hexagonal, etc. A resistance wire, wound to a given diameter and fitted with a terminal at each end, makes up the helix assembly or working part of the heating element. The helix assembly lies at the core of the sheath and runs its length, with the terminals extending past the ends of the sheath to provide for electrical connections. A powder, typically magnesium oxide, fills the space between the helix and the inside wall of the tube to serve as an electrical insulator and heat conductor. Heating elements properly annealed can be formed to the desired shape.
In general, tubular electric heating elements may be operated at temperatures to about two thousand degrees Fahrenheit. While the coil of resistance wire may reach a very high temperature, the terminal at each end remains relatively cool and is therefore known as a "cold pin". The terminals passing through the ends of the heating element typically remain in a temperature range of 150.degree. to 200.degree. F.
Seals are necessary at each end of the tubular heating element. The seals serve as an electrical insulator between the sheath and the terminal and retard or prevent the entrance of water into the heating element. Resin bushings have been used as end seals, such as in U.S. Pat. No. 4,182,948, but better sealing has been obtained with end seals formed in-situ from glass, ceramics and polymers. These formed in-situ seals can be hermetic seals or "breathing" seals.
Hermetic seals serve as a substantially impervious barrier to entry of gases and liquids at each end of the heating element, and have been formed of glass or a ceramic in the prior art, for example, in U.S. Pat. Nos. 3,195,093, 4,034,330, and 4,506,251. In addition, epoxy materials are known for use as hermetic seals, as they are thermosetting and cure to heat resistant and substantially impervious materials.
Hermetic seals, however, present a problem when used with elements having an operating temperature of 1000.degree. F. or more. Elements operating at these high temperatures consume oxygen inside the sheath by oxidation of the sheath and the wire. Once the existing supply of oxygen within the sheath is exhausted, additional oxygen consumption may take place by breakdown of the insulating material. As reported in U.S. Pat. No. 3,195,093, it is possible that a vacuum will be formed within the sheath, leading to a decrease in the thermal conductivity of the insulating material and a commensurate increase in the temperature of the wire, resulting in vaporization and failure of the resistance wire after a relatively short time.
In order to avoid the problems inherent in the use of hermetic seals with high temperature heating elements, it is also known to utilize "breathing" end seals with such heating elements. To form breathing end seals, a thermosetting silicone fluid is applied to the sheath ends, and allowed to wick into the element. When a wick depth of 1 to 3 inches occurs, heat is applied to make the fluid gel. The silicone seals are permeable to air, and allow normal oxidation to take place within the sheath.
While breathing seals do allow air to pass through to the inside of the sheath at high temperatures, they also allow water vapor to pass through to the inside of the sheath at low temperatures. Without routine operation, elements with breathing seals accumulate high levels of moisture and exhibit proportionally high current leakage between the heating element and the sheath. Thus, both hermetic and breathing seals have serious disadvantages when utilized in heating elements designed to operate at temperatures over 1000.degree. F.